Sulfur is known to possess a very high theoretical capacity of about 1672 mAh/g, and lithium-sulfur batteries in which sulfur is used as a cathode active material have been the subject of active studies. Lithium-sulfur batteries are roughly classified into liquid lithium-sulfur batteries in which the electrolyte is liquid, and all-solid-state lithium-sulfur batteries in which the electrolyte is solid.
A drawback of the liquid lithium-sulfur batteries is that lithium polysulfide generated by the reaction of lithium ions with sulfur is dissolved into the electrolyte solution, thereby adversely affecting the charge/discharge capacity and the life of the batteries.
In contrast, the all-solid-state lithium-sulfur batteries are free from the drawback that lithium polysulfide is dissolved into the electrolyte solution, and are thus suitable in maintaining the battery charge/discharge capacity and prolonging the battery life. Moreover, for example, they are free of combustible organic solvents and thus can ensure safety without the risk of electrolyte leakage or ignition, and they do not require a separator. These excellent characteristics of the all-solid-state lithium-sulfur batteries have been drawing attention. In the cathode mixture layer of the all-solid-state lithium-sulfur batteries, a reversible reaction represented by the following formula (1) occurs, wherein the reaction toward the right predominantly proceeds during discharge, and the reaction toward the left predominantly proceeds during charge.S+2Li++2e−⇄Li2S  (1)
In the all-solid-state lithium-sulfur batteries, however, since the anode, the solid electrolyte layer, and the cathode mixture layer are substantially free of solvents, and the sulfur contained as a cathode active material in the cathode mixture layer has electrical insulation properties, the cathode mixture layer has very low electron conductivity and very low lithium-ion conductivity. Accordingly, disadvantageously, the all-solid-state lithium-sulfur batteries exhibit poor reactivity in the reaction shown in the formula (1) during charge and discharge, thereby failing to ensure a sufficient charge/discharge capacity.
Patent Literature 1 discloses a cathode for all-solid lithium secondary batteries which is prepared by mechanically milling a starting material mixture containing sulfur, a carbon material having an average particle size of 100 nm or less, and an electrolyte represented by Li2S—PxSy where x and y each represent an integer that gives a stoichiometric ratio, and molding the resulting complex. According to this literature, with the cathode for all-solid lithium secondary batteries formed of the above molded product, the batteries have a high charge/discharge capacity and can be charged and discharged at a high current density.